1. Field of the Invention
This invention has to do with a method for preparing vanadium-phosphorus-zirconium catalysts with high activity and selectivity and good physical strength for the oxidation of alkanes, cycloalkanes and mixtures rich therein to dicarboxylic acid anhydrides, particularly butane to maleic anhydride.
2. Description of the Prior Art
Vanadium-phosphorus complex catalysts for the oxidation of butane to maleic anhydride, are described in U.S. Pat. No. 3,293,268. Such catalysts operate at temperatures greater than 500.degree.C. In general, yields of maleic anhydride with such catalysts are relatively low and not commercially attractive or feasible.
More recently, catalysts comprising antimony, molybdenum and iron or vanadium, have been described for oxidizing C.sub.4 and C.sub.5 paraffin hydrocarbons to maleic anhydride. The catalysts are indicated to be useful at 300.degree.-600.degree.C.
Metal-promoted vanadium-phosphorus complex catalysts are described in U.S. Pat. No. 3,156,705. The metal promoters, identified as phosphorus stabilizers, are broadly disclosed to include transition metals and rare earth metals. The catalysts are taught for oxidizing an olefin (butene) to a dicarboxylic acid anhydride (maleic anhydride). There is no teaching that such catalysts are effective in the more difficult oxidation of saturated hydrocarbons (alkanes and cycloalkanes).
In application Ser. No. 261,030, filed June 8, 1972 now abandoned, there is described an improved process for oxidizing an alkane to a dicarboxylic acid anhydride in the presence of a catalyst comprising a complex reaction product of a vanadium oxysalt and phosphoric acid promoted with one or more of Cr, Fe, Hf, Zr, La and Ce. The atomic ratio of P/V is between about 0.5 and about 2, and the atomic ratio of promoter metal/V is between about 0.0025 and about 1, in such catalysts.
As an improvement over, and an extension of, the catalysts described in said application Ser. No. 461,777, now U.S. Pat. No. 3,888,886, the present invention is concerned with a particular method for making V/P/Zr catalysts and with the particular catalysts obtained with that method. In prior catalyst preparation methods using HCl, at least 5 moles or more of HCl were required per gram atom of vanadium. Such preparation method presents a high corrosivity problem requiring the use of expensive corrosion resistant equipment. It also increases the volume of material that must be handled. The method of this invention, however, does not involve the use of hydrogen halide (HCl) and eliminates problems associated with its use. When the vanadium source is a halogen-containing vanadium salt, the amount of by-product hydrogen halide encountered is relatively and comparatively small and involves a relatively negligible problem.